I've had this problem when sharing a computer.
Have you tried this:
When you log on click the bit that says "Log on to check new messages".
Then sign in and you'll find yourself on the Inbox for your private messages. You can then go to the main page.
I don't know why, but that seems to work - or at least it's worked for me.
Hope that helps - let us know if you are still having problems.
"Goodness is about what you do. Not what you pray to."